1. Field of the Invention
The present invention relates to a liquid developer containing colored particles such as toner in the liquid and develops a latent image formed on an image carrier such as a photoconductive member by the colored particles, and also relates to a copier, facsimile apparatus, printer or similar image forming apparatus using the liquid developer.
2. Description of the Background Art
A liquid developer having toner dispersed in an organic solvent mainly composed of an isoparaffinic hydrocarbon has been conventionally known in the art, and is, for example, disclosed in JP 05-045937 A. Examples of such organic solvents which are commercially available in the market include Isopar (trade name) from Exxon Corp., Shellsol (trade name) from Shell Kagaku KK., and Soltol (trade name) from Philips Petroleum Co., Ltd.
Besides, liquid developers in which colored particles are dispersed in nonvolatile liquids have been also known in the art. For example, a nonvolatile liquid using silicone oil is disclosed in JP 12-206738 A. Furthermore, an example of a liquid-type image forming apparatus using a liquid developer is disclosed in JP 2001-305887 A.
As disclosed in JP 05-045937 A, however, the conventional liquid developer using an organic solvent mainly composed of a high-volatility isoparaffinic hydrocarbon as a liquid has a problem, that is, it is very hard to handle the isoparaffinic hydrocarbon because of its high volatility.
On the other hand, since the liquid developer disclosed in JP 12-206738 A uses a nonvolatile liquid and does not generate toxic volatile gas, the problem mentioned above can be solved. However, this kind of liquid developer tends to cause deterioration in image quality when the proportion of the liquid to colored particles is hardly adjusted in an appropriate manner in the successive steps of the image forming process including developing, transferring, and fixing steps. In the developing step, for instance, colored particles are electrophoresed from a developer carrier to a photoconductive member. If the proportion of the liquid is too small, such an electrophoresis of the colored particles can not be effectively performed. Likewise, the transferring step requires an amount of the liquid enough to electrophorese the colored particles constituting a visible image from the image carrier to a recording medium such as a sheet of transfer paper. Nevertheless, the transferring step makes the colored particles more difficult to be electrophoresed. This is because the colored particles electrophoresed are allowed to be applied to the latent image in the preceding developing step while a significant portion of the liquid remains on the developer carrier and is then isolated from a visible image after the development. Therefore, the amount of the liquid throughout the process from the developing step to the transferring step should be controlled to prevent removal of an excessive amount of liquid from the visible image.
However, at the time of fixing the visible image transferred to the recording medium with heat and pressure, absorbing an excessive amount of the liquid in the recording medium prevents the visible image from fixing on the recording image so as to lower the fixing ability. Since the liquid is nonvolatile, deterioration in fixing ability becomes unavoidable when the transfer of the visible image involves the absorption of the liquid in the recording medium. In the transferring step, while ensuring the amount of the liquid enough to allow the electrophoresis of the visible image, the liquid amount should be kept within a level at which the subsequent fixing process is effectively performed.
To solve this problem, JP 2001-305887 A proposes a liquid-type image forming apparatus in which an excessive amount of liquid is removed from a visible image prior to transfer of the visible image to a recording medium. The liquid-type image forming apparatus insures a favorable transfer characteristic by removing an excessive part of the liquid beyond an amount of liquid required for the transfer from a visible image having a large amount of liquid which affects the fixing process.
However, the inventors of the present invention have diligently studied and found that the fixation of a visible image may be affected even though the prior art liquid-type image forming apparatus is used. Specifically, when the colored particles are unevenly dispersed in the liquid developer, unevenness in development density inevitably occurs because high and low toner density portions are generated in the liquid developer. Thus, the inventors prepared a liquid developer in which toner provided as colored particles and a dispersant for facilitating the dispersion of the toner were dispersed in silicone oil provided as a nonvolatile liquid. The dispersant used is adsorbed to the surface of toner particles and the conformation thereof prevents the contact between the toner particles to allow the respective toner particles to be favorably dispersed in the liquid. When the prepared liquid developer was applied to the liquid-type image forming apparatus mentioned above to form an image, the toner particles were dispersed enough to effectively suppress unevenness in development density but the fixing ability is lowered. This is because, at the time of removing an excessive part of the liquid from a visible image prior to the transfer, the dispersant unavoidably remains in the visible image. The dispersant itself or the liquid absorbed therein deteriorated the fixing ability of the toner.